drop sonatype parent
drop sonatype parent

The parent is no longer supported and uses old plugin versions.

For keeping its functionality, we add

- 'sign-artifacts' profile (incl. a maven version check) for signing artifacts
- distributionManagement entries for (snapshot-)releases

See http://central.sonatype.org/pages/apache-maven.html

LearnLib/learnlib,LearnLib/learnlib,LearnLib/learnlib